Multiple-input, multiple-out (MIMO) antenna systems are envisaged for deployment in wireless communications systems designed according to future 3G and 4G standards. The multiple antennas will support high data rates and increase capacity and coverage for a cellular telephone systems that provide both high rate and low rate services to multiple users in a fixed bandwidth wireless channel. Multiple antenna systems exploit spatial characteristics of the channel using spatial multiplexing and transmit diversity.
In spatial multiplexing, multiple data streams are transmitted concurrently from multiple antennas and received by multiple antennas, Foschini et al., “On Limits of Wireless Communications in a Fading Environment when Using Multiple Antennas,” Wireless Pers. Commun., pp, 311–335, 1998. A number of architectures are known that attempt to achieve a theoretical capacity of the channel, Foschini et al., “Layered Space-Time Architecture for Wireless Communication in a Fading Environment When Using Multiple Antennas,” Bell Labs Tech. J., vol. 1, no. 2, pp. 41–59, 1996, Wolniansky et al., “V-BLAST: An Architecture for Realizing Very High Data Rates Over the Rich-Scattering Wireless Channel,” ISSSE, pp. 295–299, 1998 and Sellathurai eg. al., “TURBO-BLAST for Wireless Communications: Theory and Experiments,” IEEE Trans. Commun., vol. 50, no. 10, pp. 2538–2546, October 2002.
In transmit diversity, the same data stream is transmitted over multiple transmit antennas to increase a diversity order of the signal at the receiver, Anderson, “Antenna Arrays in Mobile Communications: Gain, diversity and channel capacity,” IEEE Antennas Propagat. Mag., vol. 42, pp. 12–16, April 2000.
Closed-loop (CL) diversity systems exploit information about the channel at the transmitter, Sandell, “Analytical analysis of transmit diversity in WCDMA on fading multipath channels,” IEEE Int. Symp. on Personal, Indoor and Mobile Radio Commun., 1999. Open-loop (OL) system do not exploit channel information, Tarokh et al., “Space Time Codes for High Data Rate Wireless Communication,” IEEE Trans. Inform. Theory, vol. 44, pp. 744–765, 1999, and Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Selected Areas in Commun., vol. 16, October, pp. 1451–1458, 1998.
With space time transmit diversity (STTD), a transmit diversity technique requires two transmit antennas and at least one receive antenna, 3GPP Standard TR 25.211, “Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD),” v5.4.0, June 2003. Multiple antenna solutions for higher number of transmit and receive antennas have also been described for 3GPP, “TR25.869: Transmitter diversity solutions for multiple antennas,” v1.2.0, September 2003, and “TR 25.876: Multiple Input Multiple Output (MIMO) Antennae in UTRA,” v1.3.0, February 2004.
While spatial multiplexing provides higher data rates and increases the spectral efficiency of wireless links, transmit and receive diversity make single stream transmission more reliable in time-varying wireless channels. Thereby, the range and area of acceptable coverage is increased. There exists a fundamental trade-off between achievable diversity and spatial multiplexing gains, Zheng and Tse, “Diversity and Multiplexing: A Fundamental Tradeoff in Multiple-Antenna Channels,” IEEE Transactions on Information Theory, Vol. 49, No. 5, pp. 1073–1096, May 2003.
Another issues is the number of antenna elements that user equipment (UE), e.g., a small, hand-held, cellular telephone can accommodate. Schemes based on STTD, for example, double space time transmit diversity with subgroup rate control, (DSTTD-SGRC), as described in U.S. patent application Ser. No. 10/209,306, “MIMO Systems with Rate Feedback and Space Time Transmit Diversity,” filed by Horng et al., on Jul. 31st, 2002, require that the number of receive antennas is only half the number of transmit antennas for proper decoding of the transmitted signal.
Pure transmit diversity techniques, such as maximum ratio transmission (MRT), do not place any minimum requirements on the number of receive antennas. V-BLAST and other similar techniques require at least as many receive antennas as transmit antennas. For example, for four transmit antennas, DSTTD-SGRC requires two antennas while V-BLAST requires four receive antennas, while MRT requires one receive antenna.